In a common radio network architecture a user equipment may be a mobile terminal by which a subscriber can access services offered by an operator's core network. A radio access network is the part of the network that is responsible for the radio transmission and control of the radio connection. A radio network subsystem controls a number of base stations in the radio access network. A radio network controller controls radio resources and radio connectivity within a set of cells. The base station handles the radio transmission and reception within one or more cells. A cell covers a geographical area. The radio coverage in a cell is provided by radio base station equipment at the base station site. Each cell is identified by a unique identity, which is broadcast in the cell. There may be more than one cell covering the same geographical area. A radio link is a representation of the communication between a user equipment and one cell in the radio access network. Iub/Iur interfaces are interfaces connecting the different nodes in the radio access network. The Iub interface interconnects the radio network controller to the base station. The Iur interface provides interconnection between one radio network controller and another. User data is transported on so-called transport bearers on these interfaces. Dependant on the transport network used, these transport bearers may e.g. be mapped to ATM Adaptation Layer type 2 (AAL2) connections in case of an Asynchronous Transfer Mode (ATM) based transport network or User Datagram Protocol (UDP) connections in case of an Internet Protocol IP based transport network.
A user equipment in an idle state monitors system information of base stations within range, to inform itself about candidate base stations in the service area etc. When a user equipment needs access to services, it sends a request over the Random Access CHannels (RACH) to a radio network controller via the most suitable base station, typically the one with the most favourable radio conditions. This is performed in two steps, first a preamble of the request is sent and when acknowledged by the base station, the request message is sent. Since the uplink propagation of the RACH is only approximately known, the user equipment gradually increases the transmission power of a preamble until either it has been acknowledged via the Acquisition channel (AICH), or the maximum number of attempts has been reached. The preamble and the request message are sent via the base station to the radio network controller. Upon acknowledgement, the RACH request message is sent. After admission control, the radio network controller initiates the connection via the most suitable base station if there are available resources. Uplink coverage is thus a necessity in order to successfully complete random access. The downlink AICH is commonly divided into downlink access slots, and each access slot is of length 5120 chips. Similarly, the uplink PRACH is divided into uplink access slots, each access slot is of length 5120 chips. Uplink access slot number n is transmitted from the user equipment τp-a chips prior to the reception of downlink access slot number n, n=0, 1, . . . , 14.
Transmission of downlink acquisition indicators may only start at the beginning of a downlink access slot. Similarly, transmission of uplink RACH preambles and RACH message parts may only start at the beginning of an uplink access slot.
FIG. 1 depicts the timing relation between physical RACH and AICH as seen at the user equipment according to prior art.
The preamble-to-preamble timing distance τp-p shall be larger than or equal to the minimum preamble-to-preamble.τp-p,min, i.e. τp-p≧τp-p,min.
In addition to τp-p,min, the preamble-to-Acquisition distance τp-a and preamble-to-message distance τp-m may be defined as follows:
when AICH Transmission Timing is set to 0, thenτp-p,min=15360 chips(3 access slots)τp-a=7680 chipsτp-m=15360 chips(3 access slots)when AICH Transmission Timing is set to 1, thenτp-p,min=20480 chips(4 access slots)τp-a=12800 chipsτp-m=20480 chips(4 access slots)
The parameter AICH Transmission Timing is signalled by higher layers.
The preamble is detected using energy detection relative a preamble threshold, which may be configured from the radio network controller to the base station over Node B Application Part (NBAP). A too low threshold would mistakenly trigger preambles from thermal noise, interference by others and similarly, a too high threshold will trigger preambles at very high power levels, or miss preambles all together. The threshold needs to be set considering the worst case uplink load situation.
Uplink Radio Resource Management (RRM)
The radio network controller may control resources and user mobility such as in 3GPP release 99. Resource control in this framework means admission control, congestion control, channel switching (roughly changing the data rate of a connection). Furthermore, a dedicated connection is carried over a Dedicated CHannel (DCH), which is realized as a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel).
The trend is to decentralize decision making, and in particular the control over the short term data rate of the user connection. The uplink data may then be allocated to a Enhanced DCH (E-DCH), which is realized as the triplet: a DPCCH, which is continuous, an Enhanced DPCCH (E-DPCCH) for data control and an Enhanced DPDCH (E-DPDCH) for data. The two latter are only transmitted when there is uplink data to send. Hence the base station uplink scheduler determines which transport formats each user can use over E-DPDCH. The radio network controller is however still responsible for admission control. In the Wideband Code Division Multiple Access (WCDMA) uplink, there is a trade-off between coverage and enabled peak rates. This is even more emphasized with enhanced uplink, which supports higher bit rates than ordinary dedicated channels. The uplink resources are limited by the thermal noise and interference, i.e. rise over thermal (RoT) that the cell can tolerate. The RoT limit is either motivated by coverage requirements or power control stability requirements. When only one user is connected in the cell, both power control stability and coverage are minor issues, since the uplink interference is likely to be dominated by the power generated by this user. In such a case it is tempting to allow a high RoT in order to allow high signal level over the noise (Ec/Io), which enables the use of high uplink bit rates. Conversely, in order to use the high uplink bit rates, the user connections have to provide high Ec/Io, which implies high RoT.
Cells operating at high RoT will have limited coverage, and it might be impossible to successfully complete random access from some parts of the service areas. The RACH preamble will not be detected by the system at these high RoT when sent from parts of the service area. Furthermore, the gradual power increase may generate significant interference, which could have a negative impact on the data rate of the active user. Neither is it possible to lower the preamble threshold because:                This will lead to many erroneous preamble detections from only thermal noise, which will give unnecessary Iub transmissions of subsequent RACH messages.        This will in many cases result in subsequent RACH transmissions at too low power levels, which will not be decoded correctly.        
Moreover, the short time between a received preamble and when an acquisition indicator is expected means that it is very limited time for processing before it has to be determined whether a preamble was sent at a sufficient power level.